The present invention is related to suspension systems for motor vehicles, and in particular to a control system for a vehicle suspension system having struts which contain a compressible fluid.
Prior art suspension systems have been provided for motor vehicles to isolate the vehicle frame, or chassis, from impacts and vibrations resulting from vehicle wheels traversing uneven terrain. Vehicle ride characteristics have complex dynamics characterized by nonlinearities, vehicle roll and pitch, vehicle flexibility effects, varying parameters, unknown friction, deadzones and high amplitude disturbances. Excess vibration results in artificial vehicle speed limitations, reduced vehicle-frame life, biological effects on passengers and detrimental consequences to cargo. Present automobile suspension systems traditionally use passive suspension systems which can only offer a compromise between the two conflicting criteria of comfort and performance by providing spring and dampening coefficients of fixed rates. For example, sports cars usually have stiff, harsh, performance suspensions with poor ride quality, while luxury sedans typically have softer suspensions with poor road handling capabilities. Passive suspension systems have been provided by separate coil springs and shock absorbing dampers, in which power is not input by a controlled power source to counteract impacts and vibrations resulting from traversing the rough terrain. The traditional engineering practice of designing spring and dampening functions as two separate functions has been a compromise from its inception in the late 1800s. As a result, vehicles have always been designed, styled and built around the space-weight requirements and performance limitations of traditional suspension configurations. Due to the demands of increased fuel mileage and decreased emissions, passenger and commercial vehicles are becoming lighter, which results in the differences between laden and unladen weights of the vehicles becoming so broad that traditional suspension systems are unable to span the load range effectively, causing serious degradation in performance of the vehicle ride quality, load handling and control.
Active suspension systems attempt to reduce these undesirable ride characteristics by providing active, powered components which isolate the car body from tire vibrations induced by uneven terrain, to provide improved comfort, road handling performance and safety for a variety of terrains and vehicle maneuvers. In active vehicle suspension systems, actuators are provided to actively apply forces which counteract and balance forces applied to the chassis of the motor vehicle.
Such active systems have used various control schemes to determine the amount of force which actuators should apply to the vehicle chassis to provide a smoother ride, such as schemes based on balancing the forces acting on the chassis and schemes based on supporting the vehicle chassis at a selected ride height. Active suspension systems should be able to provide different behavioral characteristics dependent upon various road conditions, without going beyond the travel limits of active suspension components. However, active systems typically require large power inputs to provide an actuator that is quick enough to compensate for impacts and vibrations which occur at desired traveling velocities over rough terrain. The power requirements for such fully active suspension systems are generally prohibitively demanding.
My prior art passive suspension systems have utilized struts, which include a cylinder that contains a compressible fluid that is not a gaseous fluid, but is instead preferably liquid. A rod extends into a cylinder and provides a fluid displacement device, such that the fluid pressure within the cylinder is increased by displacement of the compressible fluid when the rod is inserted further into the cylinder. A piston is mounted to the inward end of the rod and provides a dampening device. Such struts have effectively combined into a single unit the spring and damper functions of prior art suspension system components. Control means have also been suggested for such strut systems, in which the fluid pressure within the struts are controlled to determine spring rate coefficients for the struts.
A strut using compressible fluid is provided for supporting a suspended body relative to a support member which is subject to vibratory motion. The system includes struts which have respective cylinders with inner bores that define cylinder chambers. A compressible fluid is disposed within the cylinder chambers. Fluid displacement members are moveably extensible into each of the cylinder chambers, and respective seals sealingly engage between heads of each of the cylinders and the fluid displacement members to retain the compressible fluid within the cylinders. The struts supportively engage between the suspended body and the support member, with first ends of the struts connected by the respective fluid displacement members to the support members, and the second ends of the struts connected by the respective cylinders to the suspended body. An accumulator is pressurized by a pump and is connected to the cylinder chambers of the various struts by control valves. The control valves are operated to apply fluid pressure from the accumulator to the cylinder chambers to determine the values for the spring rate coefficients and dampening coefficients of the struts. Sensors are operatively connected to the struts for detecting positions of the suspended body relative to the support member. A data processing unit is operatively connected to the sensors and the control valves of each of the struts. The data processing unit emits control signals to the control valves to determine the amount of the compressible fluid disposed within each of the struts in response to sensed values for the relative positions between the suspended body to the support member.
The amount of the compressible fluid disposed within each of the cylinders of the struts is controlled to determine values for the respective spring rate coefficients and dampening coefficients of the struts. The spring rate and dampening coefficients are selected to apply balancing forces which are equal to the sum of several force components. The first force component is a an ideal, target force component based on the amount of force required to balance the various other forces acting upon the vehicle chassis. This target force component is summed together with several feedback loop force components, which include a comparison of the desired target strut force to an actual force applied by the strut, a comparison of velocity of the chassis relative to a selected sky hook reference datum, and a ride height error which is determined by comparing the actual vehicle ride height to a desired ride height for the vehicle. Frequency dependant filtering is also applied to decrease the spring rate coefficients in response to detected changes in relative positions between the suspended body and the support member which occur at frequencies beneath a low frequency threshold level, and to increase or decrease the dampening coefficients in response to detected changes in relative motion between the suspended body and the support member which occur at frequencies above a high frequency threshold level. The threshold frequencies are preferably determined by selected offsets from corresponding wheel frequencies. The amounts by which the spring rate and dampening coefficients are increased or decreased are preferably in proportion to the difference between the detected frequencies and the respective threshold frequencies.
The struts for use in such a suspension system preferably have pistons mounted to respective fluid displacement members to divide the cylinder chambers into two opposed chambers. The dampening pistons are moveable with the fluid displacement members within the cylinder chambers. Flow paths are provided in the pistons so that fluid flow occurs between the two opposed pressure chambers of respective ones of the cylinder chambers. The fluid mass in the two opposed chambers can be changed using a servo valve controller. The compressibility of the fluid determines a spring rate coefficient, and the flow paths through the pistons determine dampening coefficients for the struts. Thus, the struts have respective spring rate coefficients and dampening coefficients, and the capacity to vary the spring rate coefficient and the dampening coefficient all in a single unit. For high-frequency motion, the passive dampening features of the strut can be relied on, such that energy is not expended trying to generate all the strut forces by active means. Such struts offer direct replacement for traditional spring/shock absorber units as well as in-line MacPherson strut spring/shock configurations. Suspension systems using such struts can provide higher performance response speeds, greater dampening power, and adjustable spring and dampening rate coefficients to maintain effective usage of the full strut travel while accommodating variations in both static and dynamic loads.